These surprising conclusions come from a systematic review of all published studies of stretching by Ian Shrier, MD, PhD. Shrier is past president of the Canadian Academy of Sport Medicine and a researcher at SMBD-Jewish General Hospital in Montreal, Quebec, Canada.

"If strength or power is important to you, don't stretch before exercise. If you enjoy stretching, stretch after exercise, or at other times," Shrier tells WebMD in an email interview. "There are certain exceptions to these rules where performance depends only partly on force and power, and partly on aesthetics and range of motion -- a good example is ballet."

Shrier examined 23 articles on "acute stretching" -- stretching just before exercise. He found that:

22 of the 23 reports showed no benefit in terms of force, torque, or jumping height.

One study showed that stretching before exercise made for more efficient running.

Of the four articles that looked at running speed, one showed that stretching before exercise was helpful, one found it slowed runners, and two had "equivocal" results.

Shrier examined nine articles on regular stretching -- either after exercise or at some other time. He found that:

7 of the 9 reports showed a benefit.

The two reports showing no benefit looked only at running economy.

None of the reports found any harm in regular stretching.

"An acute bout of stretching will decrease pain temporarily but will not prevent injury," Shrier says. "Stretching over weeks to months will increase your force, power, and speed of running, and may prevent injury. Both an acute bout of stretching and regular stretching over weeks to months will increase range of motion."

Many Still Recommend Before-Exercise Stretching

Despite these studies, Shrier notes that many fitness experts still advise stretching before exercise. One of them is certified athletic trainer and physical therapist Michele Raya, PhD, assistant professor of physical therapy at the University of Miami in Coral Gables, Fla.

"In my opinion I do think it prevents injury. The benefits are pretty clear," Raya tells WebMD. "I do recommend to the athletes I am working with to stretch before activity. And I work with athletes at the high school, college, and professional level in running and jumping sports."

Shrier notes that the effect of acute stretching on running speed has yet to be determined. However, he finds no proof that stretching before exercise cuts injury risk.

"If you don't like stretching, don't worry about it. Just remain active and work your muscles through their full range of motion when possible -- bench press, for example," he says. "If you do like to stretch, the general recommendation is to stretch after your activity or at other times. Exceptions to this rule are when the range of motion is more important than the force or power you need from your muscles, such as aesthetics in ballet."

How to Stretch

Shrier and Raya both agree that a warm-up before stretching is essential. And while he doesn't recommend acute stretching, Shrier strongly advises warm-up before exercise. What kind of warm-up is best?

"Use the muscles you plan to use in the activity. Start slow, increase gradually," he says. "If jogging, start with walking, then jog slow and then slowly increase the speed until you are running at your regular pace -- maybe five to 15 minutes depending on your level of fitness. If you are planning to race, don't worry about tiring yourself out. Do this before the race to have the best race time. Don't stretch before."

Regular stretching improves flexibility, Shrier and Raya agree.

Raya says that people who want to improve their flexibility should stretch every day. Those comfortable with their flexibility, she says, need to stretch only three times a week. She advises athletes preparing for a specific event to be sure to stretch all the muscles they'll be using in the sport.

"It is real important to maintain the stretch for 30 seconds," Raya says. "If you hold a stretch for less than 30 seconds, there's no proof you lengthen the muscle. And if you hold it longer, you do no extra good beyond the first 30 seconds."

Raya warns that bouncing while stretching -- known as ballistic stretching -- seems particularly unhelpful. She advises against it unless a person has specifically trained to perform these kinds of stretches.